Those familiar with the design and operation of linear accelerators, or linacs, of the types now commonly used to either accelerate, decelerate or focus beams made up of relativistic particles, such as electrons, protons, etc., understand that in order to perform such desired functions, any linac structure must convert incoming radiation into slow resonances or so called modes, because only such slow modes contain longitudal electric fields that can efficiently couple energy to the relativistic particles. In order to accomplish such a conversion, a linac structure must either contain a dielectric, or the structure must be made periodic in form. The simplest such periodic structure is a simple grating.
As early as the year 1953 it was conclusively demonstrated that when particles travel over the surface of a grating, light is emitted. In view of that demonstrated fact, it seemed reasonable that the inverse of that effect could be used to make an accelerator structure for accelerating particles over the surface of a grating. By the year 1968, in an article entitled, "Laser Linac with Grating", published by Messrs. Y. Takeda and I. Matsui, in Nuclear Instrumentation Methods, Vol. 62, pp. 306, they proposed a structure geometry for such an accelerator grating. However, in 1975 it was proved that that specific proposed geometry for a grating would not operate as intended for accelerating relativistic particles. During the year 1980, the inventor of the invention disclosed herein showed that it was only for the particular grating geometry proposed by Takeda and Matsui that relativistic particles could not be accelerated. He further demonstrated that for skewed or otherwise more three dimensional grating geometries, the desired acceleration of relativistic particles could, in fact, be obtained. A discussion and mathematical demonstration of such an operable grating structure was described by the present inventor in an article entitled, "A Laser-Driven Grating Linac", published in Particle Accelerators Vol. 11, pp. 81-99 (1980). In that article it is further shown that if an accelerator grating is given a periodicity equal to one-half the wavelength of the resonance-exciting radiation applied to the grating, it will act as a true resonance "cavity" of the type which supports electromagnetic resonances that include electric components in the direction of desired relativistic particle acceleration or motion. Of course, such a grating structure is relatively open, rather than actually being like a conventional cavity or enclosed transmission line of the types wherein particles to be accelerated are passed along the axis of the enclosure.
It was shown in the above-noted article, published during 1980 by the present inventor, that such an appropriately excited accelerator grating for a linac would have particle accelerating modes that were restricted to the surface of the grating and would not radiate energy away from that surface. FIG. 1 herein illustrates such a periodic accelerator grating structure that is excited by incoming radiation having a wavelength lambda. The grating structure 1 includes a plurality of semi-cylindrical portions 2-6 that cause the accelerating modes (designated by the dashed arrows 7) to be restricted to the surface, in the manner depicted in FIG. 1. Such surface fields are known to be composed of four slow "evanescent" waves that cross the grating surface diagonally. The directions of movement of such evanescent waves are illustrated in FIG. 2, where a grating surface 1' is shown schematically as being flat, with four arrows 8-11 thereon to indicate the directions of motion of the evanescent waves. As in FIG. 1, dash arrows 7 are used in FIG. 2 to designate the accelerating modes, which are restricted to the grating surface. The present inventor explained in the above-referenced 1980 article, that the interference pattern resulting from the movement of evanescent waves across such an accelerator grating surface provides fields that are periodic not only in a desired particle beam accelerating direction, but also transverse to that desired beam direction. Accordingly, acceleration of charged particles introduced into such fields occurs within a sequence of channels across the grating surface.
It is also well known that in order to excite such surface fields with incoming radiation, some interruption in the half-lambda periodicity of the grating surface must be introduced. One example of such a modified accelerator grating surface 12 is shown in FIG. 3. That surface has three semicylindrical portions 12A, 12B and 12C, between which two smaller semi-cylindrical portions 13A and 13B are arranged to appropriately modify the half-lambda periodicity of the overall grating structure. The incoming radiation, designated schematically by the arrowed lines 14 and 15, is then impacted onto the grating as two interfering plane waves that come down on the grating surface on either side of the vertical (shown by dashed line 16).
Such known types of accelerator grating structures pose a number of significant difficulties. For example, it is known that the accelerating field for such grating structures must inevitably extend over the entire grating surface; accordingly, walls or some other suitable means are needed to appropriately confine the field to a desirably narrow strip. In addition, any solid structure used to form such an accelerating grating will be rapidly worn away by the high energy particles moving across its surface. Thus, it would be desirable to invent some means for successfully generating a grating from disposable material that could be readily and efficiently renewed without impairing the desired accelerating, decelerating or focusing functions. The invention disclosed herein provides novel solutions to both of those major problems.